The slug is the part of your URL that follows your domain. On the image above, the underlined part is the slug:
Follow these steps to change a case study’s URL/slug:
Open your case study in edit mode.
Click Settings in the navigation bar.
Choose Case Study URL from the list.
Write your new slug into the field.
Changing your slug will not affect your domain settings. Also note that if you change your URL slug, your old URL slug will stop working, so don’t forget to update your shared links everywhere!